Higher speed and higher reliability of a copying apparatus or a printer have been strictly sought in recent years. Meanwhile, the main body of the copying apparatus has started to be constructed of simpler components in various respects. As a result, performance demanded for a developer has become more sophisticated. Accordingly, unless an improvement in the performance of the developer can be achieved, a more excellent main body of the copying apparatus does not become viable nowadays.
Of the methods each involving developing an electrostatic latent image formed on an electrostatic latent image-bearing member with toner, a two-component developing method involving using a two-component developer obtained by mixing the toner with a magnetic carrier has been suitably employed in a full-color copying machine or printer required to provide high image quality. In the two-component developing method, the magnetic carrier provides the toner with a proper quantity of positive or negative charge through triboelectric charging, and the magnetic carrier carries the toner on its surface by means of the electrostatic attraction of the triboelectric charging.
Although various characteristics are demanded for the magnetic carrier and the toner constituting the two-component developer, characteristics particularly important for the magnetic carrier are, for example, proper charge-providing performance, resistance against an alternating voltage, impact resistance, wear resistance, resistance against spent toner, and developing performance.
These days, load on the developer in a developing unit has a tendency to increase. For example, a reduction in developer volume occurs in association with a reduction in size of the developing unit, or the speed when the developer is stirred increases owing to an increase in the output speed of the unit. As a result, when the life of the developer comes to close, i.e., the toner or an external additive is spent on the surface of the magnetic carrier, the charge-providing performance of the magnetic carrier is reduced.
To alleviate the problem, Patent Literature 1 proposes the following magnetic carrier. Fine particles are caused to adhere to the surface of the magnetic carrier provided with a coating layer made of a resin component. The magnetic carrier suppresses the adhesion of an external additive at the time of endurance by embedding silica fine particles in the recesses of the magnetic carrier. However, silica precludes the impartment of a dielectric characteristic to the magnetic carrier, and hence it is difficult to maintain the charge-retaining ability of the magnetic carrier particularly under high temperature and high humidity.
Further, Patent Literature 2 proposes that a high-dielectric substance be incorporated into the coating layer of a magnetic carrier to allow the magnetic carrier to maintain its developing performance and endurance stability. As the magnetic carrier uses a dielectric having a high dielectric constant, the charge-retaining ability of the magnetic carrier is improved. However, the specific gravity of the high-dielectric substance is so heavy that it is difficult to disperse the substance in the coating layer. As a result, its segregation or desorption occurs, thereby making it difficult to obtain stable quality.